Such recorders include one or more magnetic read/write heads which are fixed to the periphery of a rotary element which is coaxial with the axis of a cylindrical drum having a magnetic tape wound helically around its surface, with the tape running from a pay-out spool to a take-up spool. Information is recorded on the tape in sloping tracks which slope at an angle which is a function of the ratio between the tape speed and the head speed. There are two families of recorders of this type. In the first family, the heads are fixed to the periphery of a rotating plate which is coaxial with the drum and disposed in a slot provided in an equatorial plane of the drum with the magnetic tape that is wound helically around the drum running over the slot. In the second family, the drum comprises a stationary bottom portion and a rotary top portion having the heads fixed to its base.
The tape is put into place around the drum and is caused to run from the pay-out spool to the take-up spool by sets of wheels and/or spindles disposed respectively upstream and downstream from the drum.
The tape is driven in translation by means of at least one capstan disposed downstream from the drum.
In general, the friction between the tape and a cylindrical body (such as the drum) over which it is slipping increases the tension in the tape downstream from the body. If the tape is wound through an angle "d" around the body (as seen in plan view), if the values of the tension in the tape upstream and downstream from said body are T.sub.1 and T.sub.2 respectively, and if the coefficient of friction between the tape and the fixed body is "f", so long as: EQU T.sub.2 &lt;T.sub.1 .multidot.e.sup.fd ( 1)
the tape does not slip, and as soon as EQU T.sub.2 =T.sub.1 .multidot.e.sup.fd ( 2)
then the tape slips over the fixed body. A constant factor is omitted from equations (1) and (2), which factor is a function of the contact area between the tape and the fixed body.
The tension T.sub.2 is thus a function of the values of f and d. The value of d is generally fixed and unalterable. In contrast, the value of f may be varied for given conditions of humidity, by increasing the temperature, or the pressure, or the surface roughness of the tape, for example, and it may take values such that the tension T.sub.2 becomes excessive, preventing the tape from running properly, and possibly even damaging the physical integrity of the tape.
It is therefore important to be able to determine, or at least to control, the tension in the tape along the length thereof, and in particular downstream from the drum.
Although the tension T.sub.0 in the length of the tape at the outlet from the pay-out spool is easily controlled, e.g. by means of a tape tension sensor servo-controlling the rotation of the pay-out spool, the same is not true of the tension T.sub.2 in the length of tape downstream from the drum which is determined by a function of T.sub.0, f and d, and other parameters, depending on the type of capstan disposed downstream from the drum as provided in prior art devices.
FIG. 1A is a diagrammatic plan view of the path followed by the tape in a recorder provided with a prior art tape drive device. The tape "a" (under tension T.sub.1) coming from a pay-out spool which is not shown passes over a first deflector wheel "b.sub.1 ", and then winds round a drum "C" through an angle ".delta." with a coefficient of friction "f", then bears against a second deflector wheel "b.sub.2 " (to given tension T.sub.2), and is finally associated with a capstan of the type including a pinch wheel (and comprising a drive wheel "e.sub.1 " and a pinch wheel "e.sub.2 " with the tape being pinched therebetween). The tensions T.sub.1 and T.sub.2 respectively upstream and downstream from the drum satisfy the equation T.sub.2 =T.sub.1 .multidot.e.sup.f.delta.. T.sub.2 therefore depends directly on the coefficient of friction f, and may become too great if the value of f increases.
FIG. 1B is a diagram similar to FIG. 1A in which the capstan is of the "enveloping" type in which the tape "a" is wound around a drive wheel G through a given angle ".delta." and is driven without sliping. The tape has a tension T.sub.2 between the drum and the capstan, and a different tension T.sub.3 downstream from the capstan. T.sub.1, T.sub.2, and T.sub.3 satisfy the relationships: EQU T.sub.1 .multidot.e.sup.f.delta. =T.sub.2 &lt;T.sub.3 .multidot.e.sup.-f'.delta.' ( 3)
When the value of f increases, T.sub.2 increases in turn, but only up to a limit value at which the tape starts sliping on the drive wheel. Tape drive is then no longer ensured in satisfactory manner.
In the prior art, attempts have been made to remedy these drawbacks by a device as shown in plan view in FIG. 1C which is provided with two "enveloping" capstans, namely a first or upstream capstan (b.sub.1, H.sub.1, b.sub.2) and a second or downstream capstan (b.sub.3, H.sub.2, b.sub.4), with both capstans rotating at the same speed and both being of the enveloping type. If the values of the tension in the tape upstream and downstream from the first capstan are written T.sub.0 and T.sub.1 respectively (where T.sub.0 is the outlet tension from the pay-out spool), and if the values of the tension in the tape upstream and downstream from the second capstan are written T.sub.2 and T.sub.3 respectively, with .delta.' being the angle through which the tape is wound around either of the capstans, and f' being the coefficient of friction between the tape and the respective capstans, then the following relationships hold: EQU T.sub.0 .multidot.e.sup.f.delta. .multidot.e.sup.-f'.delta.' &lt;T.sub.2 &lt;T.sub.3 .multidot.e.sup.f'.delta.' ( 4)
If the value of f increases, then the value of T.sub.2 increases until the tape slips on the second (downstream) capstan. The tape is then no longer driven downstream from the drum, but it continues to be driven upstream from the drum by the first (upstream) capstan. Tape then begins to accumulate, thereby releasing the tape from the drum and thus reducing friction. This reduces T.sub.2 and thus allows the second capstan to drive the tape again.
Although this prior art two-capstan prior art device provides a kind of automatic regulation of tape travel, it nevertheless suffers from drawbacks.
Because of the inequality in above relationship (4) relating the values of the tensions, the angles, and the coefficients of friction, control of tape running is only relative.
Further, it is necessary for both capstans to rotate at the same speed, and therefore to provide complex and expensive synchronizing means.
Finally, the presence of two capstans complicates the structure of the recorder, takes up a relatively large amount of space, increases the cost of the assembly, and finally is difficult to make compatible with moving tape loading means.
The object of the present invention is to remedy these drawbacks by means of a tape drive device which is simple in design and in operation, and which provides good control of the tension in the tape downstream from the drum.